Remove hardcoded ES indexername

[42wim]

This makes the ISSUE_INDEXER_NAME option actually work

[lunny]

We also need to add version check on elastic search.

[codecov-commenter]

Codecov Report

Merging #12521 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #12521      +/-   ##
==========================================
- Coverage   43.51%   43.51%   -0.01%     
==========================================
  Files         642      642              
  Lines       71003    71003              
==========================================
- Hits        30899    30898       -1     
- Misses      35098    35103       +5     
+ Partials     5006     5002       -4     

Impacted Files	Coverage Δ
modules/indexer/issues/indexer.go	58.46% <100.00%> (ø)
modules/indexer/stats/queue.go	52.94% <0.00%> (-23.53%)	⬇️
modules/indexer/stats/db.go	43.47% <0.00%> (-8.70%)	⬇️
modules/git/utils.go	73.77% <0.00%> (-3.28%)	⬇️
models/unit.go	46.57% <0.00%> (-2.74%)	⬇️
modules/queue/workerpool.go	58.77% <0.00%> (-1.23%)	⬇️
routers/repo/view.go	37.33% <0.00%> (-0.65%)	⬇️
modules/git/repo.go	49.23% <0.00%> (-0.51%)	⬇️
models/gpg_key.go	55.17% <0.00%> (+0.59%)	⬆️
services/pull/pull.go	42.72% <0.00%> (+0.69%)	⬆️
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 61a1729...9da2622. Read the comment docs.